My Guinea pig Rosie is a few months old, and I noticed a hard lump about the size of a grape under her chin. She seems to be energetic and her happy self, but I can't help but worry. I also noticed just today that her poop is very soft and sticky? Some liquid also came out with it.. Here's some images:
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I don't know if they'll show or not, but I'm so scared it's something serious! My family can't afford much, and we want to avoid surgery.. My mom thinks we'd have to put her down, but I really don't want to do that, seeing as she's only a few months old.

Does anyone know what it could be? I have a vet appointment on Thursday, but I'm really curious and scared.

It's possible it's a harmless fatty lipoma: those often form in the dewlap, but usually on older pigs.

Please make sure the vet you take her to is cavy-knowledgeable. Most neighborhood dog/cat vets aren't. Guinea pigs, rabbits, and other small mammals are known as 'exotics' in vetspeak, and their care requires specialized training and knowledge that the vast majority of 'normal' vets don't have.

From what I can feel, it seems the same size, maybe even a little smaller because her skin is now looser in that area. The vet is specialized in Guinea pigs and other small animals, so I think it should be okay. I will update you on what it is tomorrow or Friday, because her appointment is tomorrow. Thank you so much for all your help! I'm hoping what she has will end up being harmless.

First a disclaimer that I recently had an experience with a tooth abscess so I may not be the most objective person.

Having said that, this sounds reminiscent of Roxy with her abscess. Is the lump movable? Does it feel like it is under the skin? Is she painful when it is manipulated? Does it feel hard and spherical? Or squishy?

Tooth involvement would be consistent with eating less and thus with change in poop output and consistency - though you note that she doesn't seem to be losing weight.

The lump is hard and spherical, and it's barely moveable. If you really tried to move it, you could, but Rosie doesn't like being touched in that area and pushes me away. I don't know if that's because of disliking being touched, or from pain.

When Roxy had her abscess, a large (we are talking 1.5-2 inch), hard, spherical lump developed under her chin seemingly over night. She was still eating pretty well. I got her in to see the vet the next day and they operated the day after.

Others who have more experience can weigh in on whether a tooth abscess sounds possible (or whether my thoughts are coloured by my recent experience). I'd say she really needs to see a vet asap, so it's good that you have an appointment for her. They may well want to do some imaging of the area.

Thank you so much for all your help! Her vet appointment is at 1:30 (it's 6:30 now). I can't be there because I have finals all day, but my mom is keeping me posted. I'll definitely catch all of you up on what it is!